Metal oxide smelting method

ABSTRACT

Provided is a smelting method in which, for example, a metal oxide such as a nickel oxide ore including nickel oxide is used as a source material and is reduced with a carbonaceous reducing agent to obtain a reduced product, with which method efficient processing can be achieved. This metal oxide smelting method is, for example, a nickel oxide ore smelting method. Specifically, the method includes a reduction process step S 3  that has: a drying step S 31  in which a mixture that was obtained by mixing a metal oxide and a carbonaceous reducing agent is dried; a preheating step S 32  in which the dried mixture is preheated; a reduction step S 33  in which the preheated mixture is reduced using a rotary hearth furnace  1,  a hearth of which rotates; and a cooling step S 35  in which the obtained reduced product is cooled.

TECHNICAL FIELD

The present invention relates to a metal oxide smelting method and relates to a smelting method in which, for example, nickel oxide ore or the like is used as a source material and is reduced with a carbonaceous reducing agent to obtain a reduced product.

BACKGROUND ART

As methods for smelting nickel oxide ore which is called limonite or saprolite, known are a dry smelting method for producing nickel matte using a flash smelting furnace, a dry smelting method for producing ferronickel using a rotary kiln or a moving hearth furnace, a HPAL process that is a hydrometallurgical method for obtaining nickel-cobalt mixed sulfide (mixed sulfide) using an autoclave by adding a high pressure acid leach sulfating agent, and the like.

Among the various methods mentioned above, particularly, in a case where nickel oxide ore is reduced and smelted using a dry smelting method, a treatment for forming nickel oxide ore of a source material into a lump product by crushing the nickel oxide ore into a proper size and the like, a treatment for forming a slurry, or the like is performed as a pretreatment.

Specifically, when nickel oxide ore is formed into a limp product, that is, a lump is formed from a powdery or granular ore, it is general that the nickel oxide ore is mixed with other components, for example, a binder or a reducing agent such as coke to obtain a mixture and the mixture is further subjected to moisture adjustment and the like, then charged into a lump product manufacturing machine, and formed into a lump product referred to as a pellet or a briquette (hereinafter, collectively simply referred to as the “pellet”) having, for example, one side or a diameter of about 10 mm to 30 mm.

Further, the pellet is required to exhibit gas permeability to a certain degree in order to “emit” the moisture contained. Furthermore, the composition of the reduced product to be obtained is non-uniform and a trouble that metal is dispersed or unevenly distributed is caused when the reduction does not uniformly proceed in the pellet. For this reason, it is important to uniformly mix the mixture and maintain the temperature as constant as possible when the pellet is subjected to the reduction treatment.

In addition, it is also a significantly important technique to coarsen the ferronickel to be generated by reduction. The reason for this is that, in a case where the generated ferronickel has a fine size of, for example, several tens of μm to several hundreds of μm or less, it is difficult to separate the ferronickel from slag to be generated at the same time, and thus a recovery rate (yield) as ferronickel greatly decreases. For this reason, a treatment for coarsening the reduced ferronickel is required.

Further, it is also an important technical problem how the smelting cost can be suppressed low, and a continuous treatment that can be operated in a compact facility is desired.

For example, Patent Document 1 discloses a technique relating to a method for producing ferronickel, and particularly to a method for producing ferronickel or a source material for smelting ferronickel from nickel oxide ore of a low grade with high efficiency. Specifically, disclosed is a method including a mixing step of mixing a source material containing nickel oxide and iron oxide with a carbonaceous reducing material to obtain a mixture, a reduction step of reducing the mixture in a moving hearth furnace by heating to obtain a reduced mixture, and a melting step of melting the reduced mixture in a melting furnace to obtain ferronickel.

Herein, Patent Document 1 describes that by setting the metallized rate of Ni in the reduced mixture to 40% or more, preferably 85% or more, heat required for reducing nickel oxide remaining in the reduced mixture in the melting furnace is decreased so that energy consumption in the melting furnace can be reduced. However, even if the heat required for the reduction in the melting furnace is decreased by increasing the metallized rate of Ni in the reduced mixture (hereinafter, also referred to as the “metallized rate”), the heat quantity itself required for metallizing Ni is the same, the energy consumption is not reduced when considered as a whole, and accordingly, the smelting cost is not reduced.

Further, Patent Document 1 describes that a reduced lump product (reduced mixture) reduced in the moving hearth furnace is generally cooled to about 1000° C. with, for example, a radiant cooling plate or a refrigerant spraying machine provided in the moving hearth furnace and then is discharged with a discharger. However, upon the reduced lump product is cooled to about 1000° C. or lower and discharged and recovered from the moving hearth furnace, the moving hearth furnace is cooled, and energy for increasing the temperature again for the reduction is needed, which incurs cost. Further, when cooling and heating are repeated, thermal shock to the furnace increases to shorten the life span of the device, which also causes an increase in cost.

As described above, there are many problems in order to obtain ferronickel by mixing and reducing nickel oxide ore and continuously performing smelting at low cost.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in view of such circumstances, and provides a smelting method in which, for example, a metal oxide such as nickel oxide ore including nickel oxide or the like is used as a source material and is reduced with a carbonaceous reducing agent to obtain a reduced product, with which method an efficient treatment can be achieved.

Means for Solving the Problems

The present inventors have conducted intensive investigations to solve the above-mentioned problems. As a result, it has been found that an efficient smelting treatment can be performed by subjecting a mixture containing a source material of a metal oxide to a reduction treatment in which a drying step, a preheating step, a reduction step using a rotary hearth furnace, and a cooling step are sequentially performed, whereby the present invention has been completed.

(1) A first invention of the present invention is a metal oxide smelting method including a reduction treatment step including: a drying step in which a mixture obtained by mixing a metal oxide and a carbonaceous reducing agent is dried; a preheating step in which the dried mixture is preheated; a reduction step in which the preheated mixture is reduced using a rotary hearth furnace, a hearth of which rotates; and a cooling step in which the obtained reduced product is cooled.

(2) A second invention of the present invention is the metal oxide smelting method in the first invention, in which the reduced product obtained through the reduction step is subjected to a temperature maintenance step in which the reduced product is maintained at a prescribed temperature in the rotary hearth furnace, and after maintained for a prescribed time, the reduced product is supplied to the cooling step.

(3) A third invention of the present invention is the metal oxide smelting method in the second invention, in which the rotary hearth furnace has a structure capable of partitioning the respective steps, and a treatment in the reduction step and a treatment in the temperature maintenance step are performed using the same rotary hearth furnace.

(4) A fourth invention of the present invention is the metal oxide smelting method in the second or third invention, in which the reduced product is maintained at a temperature of 1300° C. or higher and 1500° C. or lower in the temperature maintenance step.

(5) A fifth invention of the present invention is the metal oxide smelting method in any one of the first to fourth inventions, in which in the reduction step, reduction is performed while a reducing temperature is set to 1200° C. or higher and 1450° C. or lower.

(6) A sixth invention of the present invention is the metal oxide smelting method in any one of the first to fifth inventions, in which the mixture to be dried in the drying step is obtained through a mixing treatment step in which at least a metal oxide and a carbonaceous reducing agent are mixed to obtain a mixture, and a pretreatment step in which a treatment of forming the obtained mixture into a lump product or a treatment of filling the mixture in a prescribed container is performed.

(7) A seventh invention of the present invention is the metal oxide smelting method in any one of the first to sixth inventions, further including a separating step in which the reduced product cooled in the cooling step in the reduction treatment step is separated into a metal and slag and the metal is recovered.

(8) An eighth invention of the present invention is the metal oxide smelting method in any one of the first to seventh inventions, in which the metal oxide is nickel oxide ore.

(9) A ninth invention of the present invention is the metal oxide smelting method in any one of the first to eighth inventions, in which the reduced product contains ferronickel.

Effects of the Invention

According to the present invention, it is possible to provide a smelting method in which, for example, a metal oxide such as nickel oxide ore including nickel oxide or the like is used as a source material and is reduced with a carbonaceous reducing agent to obtain a reduced product, with which method an efficient treatment can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process diagram illustrating an example of the flow of a method for smelting nickel oxide ore.

FIG. 2 is a process diagram illustrating a treatment step to be performed in a reduction treatment step.

FIG. 3 is a diagram (plan view) illustrating a configuration example of a rotary hearth furnace, a hearth of which rotates.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

<<1. Overview of Present Invention>>A metal oxide smelting method according to the present invention is a smelting method in which a metal oxide is used as a source material and a reduction treatment is performed by a carbonaceous reducing agent at a high temperature to obtain a reduced product. For example, there is mentioned a method for producing ferronickel by using nickel oxide as a metal oxide or nickel oxide ore including iron oxide or the like as a source material and reducing the source material for smelting using a carbonaceous reducing agent under a high temperature.

Specifically, the metal oxide smelting method according to the present invention includes reduction treatment step that includes a drying step in which a mixture obtained by mixing a metal oxide and a carbonaceous reducing agent is dried, a preheating step in which the dried mixture is preheated, a reduction step in which the preheated mixture is reduced using a rotary hearth furnace, a hearth of which rotates, and a cooling step in which the obtained reduced product is cooled.

In this manner, according to the present invention, the mixture containing metal oxide as a source material is subjected to the treatment in each step mentioned above, and further the treatment in the reduction step is performed using the rotary hearth furnace, a hearth of which rotates, so that the metal contained in the metal oxide can be effectively metallized and an efficient smelting treatment can also be performed.

Hereinafter, as a specific embodiment of the present invention (hereinafter, referred to as the “present embodiment”), a method for smelting nickel oxide ore will be described as an example. The nickel oxide ore serving as a source material for smelting contains at least nickel oxide. In this method for smelting nickel oxide ore, ferronickel (iron-nickel alloy) can be produced by reducing nickel oxide and the like contained in the source material.

Incidentally, in the present invention, the metal oxide is not limited to nickel oxide ore and the smelting method is also not limited to a method for producing ferronickel from nickel oxide ore containing nickel oxide and the like. Further, various modifications can be made without changing the gist of the present invention.

<<2. Method for Smelting Nickel Oxide Ore>>

The method for smelting nickel oxide ore according to the present embodiment is a method for generating ferronickel, which is a metal, and slag by mixing and kneading nickel oxide ore serving as a source material for smelting with a carbonaceous reducing agent or the like to obtain a mixture and subjecting the mixture to a reduction treatment. Incidentally, ferronickel, which is a metal, can be recovered from a mixture containing a metal and slag obtained through the reduction treatment by separating the metal.

FIG. 1 is a process diagram illustrating an example of the flow of a method for smelting nickel oxide ore. As illustrated in FIG. 1, this method for smelting nickel oxide ore includes a mixing treatment step S1 in which nickel oxide ore and a material such as a carbonaceous reducing agent are mixed to obtain a mixture, a reduction charging pretreatment step S2 in which the obtained mixture is formed into a lump product or filled in a prescribed container, a reduction treatment step S3 in which the mixture is reduced at a prescribed temperature (reducing temperature), and a separating step S4 in which the metal is separated and recovered from the mixture containing the metal and slag generated by the reduction treatment.

<2-1. Mixing Treatment Step>

The mixing treatment step S1 is a step in which source material powders including nickel oxide ore are mixed to obtain a mixture. Specifically, in the mixing treatment step S1, nickel oxide ore serving as a source material for smelting and source material powders, such as iron ore, a flux component, a binder, or a carbonaceous reducing agent, having a particle diameter of, for example, about 0.2 mm to 0.8 mm are mixed at a prescribed ratio to obtain a mixture.

The nickel oxide ore serving as an ore of a source material for smelting is not particularly limited, but limonite ore, saprolite ore, and the like can be used.

As the iron ore, for example, iron ore having an iron grade of about 50%, hematite to be obtained by hydrometallurgy of nickel oxide ore, and the like can be used.

An example of compositions (% by weight) of nickel oxide ore serving as a source material and iron ore is presented in the following Table 1. Incidentally, the composition of the source material is not limited thereto.

TABLE 1 Source material [% by weight] Ni Fe₂O₃ C Nickel oxide ore 1~2 50~60 — Iron ore — 80~95 —

Further, examples of the binder may include bentonite, a polysaccharide, a resin, water glass, and dehydrated cake. Further, examples of the flux component may include calcium oxide, calcium hydroxide, calcium carbonate, and silicon dioxide.

The carbonaceous reducing agent is not particularly limited, and examples thereof include coal powder and coke. Incidentally, this carbonaceous reducing agent preferably has a size equivalent to the particle size of the nickel oxide ore of the source material ore. Further, the amount of the carbonaceous reducing agent mixed can be adjusted such that the proportion of carbon amount is 5% or more and 60% or less when the total value (also conveniently referred to as the “total value of chemical equivalents”) of a chemical equivalent required for reducing the entire amount of nickel oxide contained in the mixture to be formed into nickel metal and a chemical equivalent required for reducing ferric oxide contained in the pellet into iron metal is regarded as 100%.

In the mixing treatment step S1, a mixture is obtained by uniformly mixing source material powders including nickel oxide ore as described above. Upon this mixing, kneading may be performed at the same time as mixing or kneading may be performed after mixing. In this manner, by mixing and kneading the source material powders, the contact area between the source materials increases and by decreasing voids, the reduction reaction is likely to occur and the reaction can be uniformly conducted. Accordingly, the reaction time of the reduction reaction can be shortened and variation in the quality is diminished. As a result, a highly productive treatment can be performed and high quality ferronickel can be produced.

Further, after kneading the source material powders, the mixture may be extruded using an extruder. By extruding the mixture using an extruder in this manner, a still higher kneading effect can be obtained, the contact area between the source material powders increases, and voids can be decreased. Accordingly, high quality ferronickel can be efficiently produced.

<2-2. Reduction Charging Pretreatment Step (Pretreatment Step)>

The reduction charging pretreatment step S2 is a step in which the mixture obtained in the mixing treatment step S1 is formed into a lump product or filled in a container. That is, in this reduction charging pretreatment step S2, the mixture obtained by mixing the source material powders is molded such that the mixture is easily charged into a furnace used in the reduction treatment step S3 described later and the reduction reaction efficiently occurs.

(Forming Mixture into Lump Product)

In the case of forming the obtained mixture into a lump product, the mixture is formed (granulated) into a lump product. Specifically, moisture is added to the obtained mixture in a prescribed amount required for forming the mixture into a lump product and the mixture is molded into a lump (hereinafter, also referred to as the “pellet”) using, for example, a lump product manufacturing apparatus (such as a tumbling granulator, a compression molding machine, or an extrusion molding machine) or the like.

The shape of the pellet is not particularly limited, and the shape of the pellet can be, for example, a spherical shape. By adopting a spherical pellet, the reduction reaction easily proceeds relatively uniformly, which is preferable. Further, the size of the lump product to be formed into a pellet is not particularly limited, but for example, the size (the diameter in the case of a spherical pellet) of the pellet to be charged into a smelting furnace, which is used for performing the reduction treatment (reduction step S33), through the drying treatment (drying step S31) and the preheating treatment (preheating step S32) can be set to about 10 mm to 30 mm. Incidentally, the reduction step and the like will be described in detail later.

(Filling of Mixture in Container)

In the case of filling the obtained mixture in a container, the mixture can be filled in a prescribed container while being kneaded by an extruder or the like. In this manner, after the mixture is filled in the container, the reduction treatment may be performed in the subsequent step of the reduction treatment step S3 without any changes, but the mixture filled in the container is compressed by a press or the like. By compressing and molding the mixture in the container, the density of the mixture can be increased, the density becomes uniform, the reduction reaction easily proceeds more uniformly, and ferronickel having small variation in the quality can be produced.

The shape of the mixture to be filled in the container is not particularly limited, but for example, the shape is preferably a rectangular parallelepiped shape, a cubic shape, a cylindrical shape, or the like. Further, the size thereof is not also particularly limited, but for example, if the shape is a rectangular parallelepiped shape or a cubic shape, the inside dimensions of the height and the width is preferably approximately 500 mm or less. With such a shape and such a size, the variation in the quality becomes small and highly productive smelting can be performed.

<2-3. Reduction Treatment Step>

In the reduction treatment step S3, the source material powders are mixed in the mixing treatment step S1 and the mixture formed into a lump product or filled in the container in the reduction charging pretreatment step S2 is reduced and heated at a prescribed reducing temperature. By the reduction and heat treatment of the mixture in the reduction treatment step S3, the smelting reaction proceeds and a metal and slag are generated.

FIG. 2 is a process diagram illustrating a treatment step to be performed in the reduction treatment step S3. As illustrated in FIG. 2, the reduction treatment step S3 in the present embodiment includes a drying step S31 in which the mixture is dried, a preheating step S32 in which the dried mixture is preheated, a reduction step S33 in which the mixture is reduced, and a cooling step S35 in which the obtained reduced product is cooled. Further, preferably, the reduction treatment step S3 includes a temperature maintenance step S34 in which the reduced product obtained through the reduction step S33 is maintained in a prescribed temperature range.

Herein, the treatment in the reduction step S33 is performed using a rotary hearth furnace, a hearth of which rotates. Further, in a case where the temperature maintenance step S34 in which the reduced product is maintained in a prescribed temperature range is performed, at least the treatment in the reduction step S33 and the treatment in the temperature maintenance step S34 are performed in the rotary hearth furnace.

In this manner, by performing those treatments in the rotary hearth furnace, the temperature in the rotary hearth furnace can be maintained at a high temperature, so that it is unnecessary to increase or decrease the temperature every time the treatments in the respective steps are performed, and energy cost can be considerably reduced. Further, according to the treatment using the rotary hearth furnace, the control or management of the temperature is easily conducted. According to these, high quality ferronickel can be continuously stably

produced at high productivity. (1) Drying Step

In the drying step S31, the mixture obtained by mixing the source material powders is subjected to the drying treatment. This drying step S31 is mainly intended to extract moisture or crystalline water in the mixture.

-   -   A large amount of moisture or the like is contained in the         mixture obtained in the mixing treatment step S1, the moisture         evaporates and expands at a time by a sharp increase to a high         temperature like a reducing temperature at the time of the         reduction treatment in this state, the mixture formed into a         lump product is broken, or depending on the cases, is broken         into fragments, and thus it is difficult to perform a uniform         reduction treatment. For this reason, before the reduction         treatment is performed, the mixture is subjected to the drying         treatment to remove moisture so that breakage of the pellet or         the like is prevented.

The drying treatment in the drying step S31 is preferably performed in the form of being connected to a rotary hearth furnace. The drying treatment is also considered to be performed while an area in which the drying treatment is conducted (drying area) is provided in the rotary hearth furnace, but in such a case, the drying treatment in the drying area is limited so that the treatment in the reduction step S33 and the treatment in the temperature maintenance step S34 may be affected.

Therefore, it is preferable that the drying treatment in the drying step S31 is performed in a drying chamber which is provided outside the furnace of the rotary hearth furnace and is connected to the rotary hearth furnace. Incidentally, although will be described in detail later, FIG. 3 illustrates a configuration example of a rotary hearth furnace 1 and a drying chamber 20 connected to the rotary hearth furnace 1. In this manner, by the drying chamber 20 being provided outside the furnace of the rotary hearth furnace 1, a drying chamber can be designed quite separately from steps such as preheating, reduction, and cooling described later, and desirable drying, preheating, reduction, and cooling treatments are easily performed, respectively. For example, in a case where a large amount of moisture remains in the mixture depending on the source material, since it takes time to perform the drying treatment, the entire length of the drying chamber 20 may be designed to be long or the transfer speed of the mixture in the drying chamber 20 may be designed to be decreased.

As the drying treatment in the drying chamber 20, for example, a treatment can be performed so that the solid content in the mixture is about 70% by weight and the moisture is about 30% by weight. Further, the drying method is not particularly limited, but the drying can be performed by blowing hot air to the mixture transferred in the drying chamber 20. Further, the drying temperature is not particularly limited, but from the viewpoint that the reduction reaction is not started, the drying temperature is preferably set to 500° C. or lower and it is preferable to perform uniform drying at the temperature of 500° C. or lower. An example of the composition (parts by weight) of the solid content in the drying-treated mixture is presented in the following Table 2. Incidentally, the composition of the mixture is not limited thereto.

TABLE 2 Composition of solid content in dried mixture (pellet) [Parts by weight] Ni Fe₂O₃ SiO₂ CaO Al₂O₃ MgO Binder Others 0.5~1.5 50~60 8~15 4~8 1~6 2~7 About 1 Balance

(2) Preheating Step

In the preheating step S32, the mixture after the moisture has been removed by the drying treatment in the drying step S31 is preheated (preliminarily heated).

When the mixture is charged into the rotary hearth furnace and heated rapidly to a high temperature of a reducing temperature, the mixture cracks due to thermal stress or becomes powders. Further, the temperature of the mixture does not uniformly increase, variation in the reduction reaction occurs, and the quality of the metal to be generated may deteriorate. Therefore, it is preferable that the mixture is preheated to a prescribed temperature after the mixture is subjected to the drying treatment, and according to thus, the breakage of the mixture and the variation in reduction reaction can be suppressed.

The preheating treatment in the preheating step S32 is preferably performed in the treating chamber provided outside the furnace of the rotary hearth furnace, similarly to the drying treatment, and the preheating treatment is preferably performed in a preheating chamber connected to the rotary hearth furnace. Incidentally, FIG. 3 illustrates a configuration example of a preheating chamber 30 connected to the rotary hearth furnace 1, and the preheating chamber 30 is provided outside the furnace of the rotary hearth furnace 1 and is provided continuously from the drying chamber 20 in which the drying treatment is performed. In this manner, by performing the preheating treatment in the preheating chamber 30 provided outside the furnace of the rotary hearth furnace 1, the temperature in the rotary hearth furnace 1 in which the reduction treatment is performed can be maintained to be a high temperature and energy required for heating can be considerably saved.

The preheating treatment in the preheating chamber 30 is not particularly limited, but it is preferable to perform the preheating treatment while a preheating temperature is set to 600° C. or higher and it is more preferable to perform the preheating treatment while a preheating temperature is set to 700° C. or higher and 1280° C. or lower. By performing the treatment at a preheating temperature in such a range, energy required for reheating to the reducing temperature in the subsequent reduction treatment can be considerably reduced.

(3) Reduction Step

In the reduction step S33, the mixture preheated in the preheating step S32 is subjected to a reduction treatment at a prescribed reducing temperature. Specifically, the reduction treatment in the reduction step S33 is performed using a rotary hearth furnace, a hearth of which rotates. In this manner, by performing the reduction treatment using the rotary hearth furnace, the temperature in the furnace can be maintained in a high temperature range, it is unnecessary to increase or decrease the temperature, and energy cost can be considerably reduced. Further, the control or management of the temperature is easily conducted and high quality ferronickel can be stably generated.

[Configuration of Rotary Hearth Furnace]

Herein, FIG. 3 is a diagram (plan view) illustrating a configuration example of a rotary hearth furnace, a hearth of which rotates. As illustrated in FIG. 3, the rotary hearth furnace 1 has a region 10 in which the hearth rotates and the region 10 is divided into four so that respective divided regions form treating chambers (10 a, 10 b, 10 c, and 10 d). Specifically, in this rotary hearth furnace 1, for example, all of the four treating chambers denoted by Reference Numeral “10 a” to “10 d” can be used as reducing chambers in which the reduction treatment is performed. Further, in the case of performing the temperature maintenance step S34 described later after the treatment in the reduction step S33, for example, the treating chambers “10 a, ” “10 b, ” and “10 c” can be used as reducing chambers and the treating chamber “10 d” can be used as a temperature maintaining chamber in which the treatment in the temperature maintenance step S34 is performed.

In order to strictly control the reaction temperature so as to suppress energy loss, a configuration in which the respective steps, that is, the respective treating chambers are partitioned by a partition wall is preferably employed. In this manner, according to the rotary hearth furnace having a structure capable of partitioning the respective steps, as described later, the treatment in the reduction step S33 and the treatment in the temperature maintenance step S34 can be performed using the same rotary hearth furnace while energy loss is suppressed. However, if the partition wall is a fixed type, there is a possibility that transferring between the steps, and particularly, charging and discharging to and from the rotary hearth furnace become difficult, so that the partition wall preferably has a structure which can open and close to the degree that does not affect the movement of a material to be treated.

Incidentally, the number of treating chambers formed by dividing the region 10 in which the hearth rotates is not limited to four illustrated in FIG. 3. Further, the number of reducing chambers and the number of temperature maintaining chambers are also not limited to the aforementioned examples and can be appropriately set depending on treatment time or the like.

The rotary hearth furnace 1 includes, as described above, a hearth which rotationally moves on the plane, and when the hearth on which the mixture is placed rotationally moves at a prescribed speed, the mixture passes through the respective treating chambers (10 a, 10 b, 10 c, and 10 d) and the treatment is performed at the time of the mixture passing through the treating chambers. Incidentally, an arrow on the rotary hearth furnace 1 in FIG. 3 indicates a rotation direction of the hearth and indicates a moving direction of a material to be treated (mixture).

Further, the rotary hearth furnace 1 is connected to the drying chamber 20 and the preheating chamber 30 which are provided outside the furnace, and as described above, after the mixture is subjected to the drying treatment in the drying chamber 20, the dried mixture is transferred to the preheating chamber 30 and subjected to the preheating treatment and the preheating-treated mixture is sequentially transferred to the inside of the rotary hearth furnace 1. Further, the rotary hearth furnace 1 is connected to a cooling chamber 40 provided outside the furnace, and the reduced product obtained through the reducing chamber or the temperature maintaining chamber (10 d) is subjected to the cooling treatment in the cooling chamber 40 (cooling step S35 described later).

[Reduction Treatment in Rotary Hearth Furnace]

In the reduction treatment using the rotary hearth furnace 1, it is preferable that nickel oxide, which is a metal oxide contained in nickel oxide ore, is completely reduced as much as possible; meanwhile, iron oxide, which is derived from iron ore or the like mixed as source material powder with nickel oxide ore, is partially reduced such that ferronickel having a target nickel grade is obtainable.

Specifically, the reducing temperature is not particularly limited, but is preferably set in a range of 1200° C. or higher and 1450° C. or lower and more preferably set in a range of 1300° C. or higher and 1400° C. or lower. By performing reduction in such a temperature range, the reduction reaction can uniformly occur, and metal (ferronickel metal) in which variation in the quality is suppressed can be generated. Further, more preferably, when reduction is conducted at a reducing temperature in a range of 1300° C. or higher and 1400° C. or lower, a desired reduction reaction can occur in a relatively short time.

Upon the reduction treatment, the internal temperature of the reducing chamber in the rotary hearth furnace 1 is increased to a reducing temperature in the aforementioned range, and after the temperature increases, the temperature at this time is maintained.

Further, in the reduction treatment, in order to prevent the mixture sample from being difficult to recover when the mixture sample reacts with the hearth and is not peeled off, a reaction suppressing material such as ash may be spread on the hearth of the rotary hearth furnace 1 to be used and the mixture sample may be placed on the reaction suppressing material. For example, as the ash serving as the reaction suppressing material, ash having SiO₂ as a main component and containing a small amount of an oxide such as Al₂O₃ or MgO as other components can be used.

(4) Temperature Maintenance Step

Although not an essential aspect, the temperature maintenance step S34 in which the reduced product obtained through the reduction step S33 is maintained in a prescribed high temperature condition in the rotary hearth furnace may be performed. In this manner, when the reduced product obtained by the reduction treatment at a prescribed reducing temperature in the reduction step S33 is not cooled immediately but is maintained in a high temperature atmosphere, a metal component generated in the reduced product can be settled and coarsened.

In a case where the metal component in the reduced product is small in the state of being obtained by the reduction treatment, for example, in a case where the metal component is a bulk-form metal having a size of about 200 μm or less, it is difficult to separate the metal and the slag in the subsequent separating step S4. Therefore, as necessary, by the reduced product being maintained at a high temperature over a certain time continuously after the reduction reaction is finished, the metal in the reduced product having a larger specific gravity than that of the slag is settled and aggregated to coarsen the metal.

Incidentally, in a case where the metal is coarsened to a level that does not cause any problem in production by the reduction treatment in the reduction step S33, particularly, this temperature maintenance step S34 is not necessarily provided.

Specifically, the maintaining temperature of the reduced product in the temperature maintenance step S34 is preferably set in a high temperature of 1300° C. or higher and 1500° C. or lower. By maintaining the reduced product at a high temperature in such a range, the metal component in the reduced product can be efficiently settled to obtain coarse metal. Incidentally, when the maintaining temperature is lower than 1300° C., since a large part of the reduced product becomes a solid phase, the metal component is not settled, or even if the metal component is settled, it takes time for that, which is not preferable. On the other hand, when the maintaining temperature is higher than 1500° C., reaction between the obtained reduced product and a hearth material proceeds, so that there is a case where the reduced product cannot be recovered or the furnace is damaged.

Herein, the treatment in the temperature maintenance step S34 is performed in the rotary hearth furnace 1 used in the reduction step S33 continuously to the reduction treatment. That is, as described using FIG. 3, in the rotary hearth furnace 1, for example, the treating chambers “10 a, ” “10 b, ” and “10 c” are used as reducing chambers, the treating chamber “10 d” is used as a temperature maintaining chamber in which the treatment is performed in the temperature maintenance step S34, and the reduced product obtained by passing through the reducing chambers (10 a, 10 b, and 10 c) is maintained in a prescribed temperature range in the temperature maintaining chamber (10 d).

In this manner, by continuously performing, using the rotary hearth furnace 1, the treatment in which the reduced product obtained through the reduction treatment is maintained at a prescribed temperature, the metal component in the reduced product can be efficiently settled and coarsened. Moreover, when the treatment in the reduction step S33 and the treatment in the temperature maintenance step S34 are not performed in separate furnaces but performed continuously using the rotary hearth furnace 1, heat loss between the respective treatments can be reduced and an efficient operation can be performed.

(5) Cooling Step

In the cooling step S35, the reduced product obtained through the reduction step S33 or the reduced product maintained at a high temperature over a prescribed time in the temperature maintenance step S34 is cooled to a temperature at which the reduced product can be separated and recovered in the subsequent separating step S4.

Since the cooling step S35 is a step in which the reduced product obtained as mentioned above is cooled, the cooling step S35 is preferably performed in the cooling chamber connected to the outside of the furnace of the rotary hearth furnace 1. Incidentally, although FIG. 3 illustrates the configuration example of the cooling chamber 40 connected to the rotary hearth furnace 1, this cooling chamber 40 is provided to be connected to the outside of the furnace of the rotary hearth furnace 1. In this manner, by performing the cooling treatment in the cooling chamber 40 provided outside the furnace of the rotary hearth furnace 1, the internal temperature of the rotary hearth furnace 1 can be prevented from decreasing, and energy loss can be suppressed. According to this, efficient ferronickel generation can be conducted.

The temperature in the cooling step S35 (hereinafter, also referred to as the “temperature at the time of recovering”) is a temperature at which the reduced product is handled substantially as a solid, and the temperature is preferably a high temperature as much as possible. By increasing the temperature at the time of recovering as much as possible, even when the hearth, which rotationally moves, of the rotary hearth furnace 1 returns to the connect place with the preheating chamber 30 in which the preheating step S32 is performed, energy loss can be reduced, and energy required for reheating can be still more saved. Specifically, the temperature at the time of recovering is preferably set to 600° C. or higher. By setting the temperature at the time of recovering to a high temperature in this manner, energy required for reheating can be considerably reduced, and an efficient smelting treatment can be performed at low cost. Further, by decreasing a difference in temperature inside the rotary hearth furnace 1, thermal stress to be applied to the hearth, the furnace wall, or the like can be decreased, and the life span of the rotary hearth furnace 1 can be largely expanded. Furthermore, failures during operation can also be considerably decreased.

<2-4. Separating Step>

In the separating step S4, the metal (ferronickel metal) is separated and recovered from the reduced product generated in the reduction treatment step S3. Specifically, in the separating step S4, the metal phase is separated and recovered from a mixed product (reduced product) which contains a metal phase (metal solid phase) and a slag phase (slag solid phase) which is obtained by the reduction and heat treatment of the mixture.

As a method for separating the metal phase and the slag phase from the mixed product which is composed of the metal phase and the slag phase and is obtained as a solid, for example, methods such as separation by specific gravity and separation by magnetic force can be utilized in addition to removal of unnecessary substances by sieving. Further, the metal phase and the slag phase thus obtained can be easily separated since these exhibit poor wettability, and it is possible to easily separated the metal phase and the slag phase from the mixed product by imparting an impact to the large mixed product, for example, falling down the large mixed product at a prescribed falling distance or applying a prescribed vibration to the large mixed product at the time of sieving.

The metal phase is recovered by separating the metal phase and the slag phase in this manner, and thus a product of ferronickel can be obtained.

EXAMPLES

Hereinafter, the present invention will be described in more detail by means of Examples, but the present invention is not limited to the following Examples at all. (Mixing Treatment Step)

Nickel oxide ore serving as a source material ore, iron ore, silica sand and limestone which are flux components, a binder, and coal powder which is a carbonaceous reducing agent (carbon content: 85% by weight, average particle diameter: about 190 μm) were mixed using a mixer while adding an appropriate amount of water to obtain a mixture. Incidentally, the carbonaceous reducing agent was contained in an amount corresponding to the amount of carbon of 33% when the total value of a chemical equivalent required for reducing nickel oxide and iron oxide (Fe₂O₃) into metal without being in excess or short was regarded as 100%.

Then, the mixture obtained by mixing with the mixer was kneaded by a biaxial kneader.

(Reduction Charging Pretreatment Step)

Then, the mixture obtained by kneading was classified into nine, and each mixture sample was molded using a pan granulator into a spherical pellet of φ19 ±1.5 mm.

(Reduction Treatment Step)

Then, the respective mixture samples classified into nine were subjected to a reduction treatment using a rotary hearth furnace 1 as illustrated in FIG. 3 while treatment conditions were changed. As the rotary hearth furnace 1, as illustrated in FIG. 3, a hearth furnace in which a drying chamber 20 drying the pellet, a preheating chamber 30 provided continuously to the drying chamber 20, and a cooling chamber 40 cooling a reduced product obtained through treating chambers 10 a to 10 d in the furnace are connected to the outside of the furnace was used.

Specifically, nine pellet samples were charged into the drying chamber 20 connected to the outside of the furnace of the reduction hearth furnace 1 and then subjected to a drying treatment. The drying treatment was performed in a nitrogen atmosphere substantially not containing oxygen by blowing hot air set at 250° C. to 350° C. to the pellets such that the solid content in the pellet would be about 70% by weight and the moisture would be about 30% by weight. The solid content compositions (excluding carbon) of the drying-treated pellet are presented in the following Table 3.

TABLE 3 Composition of solid content in dried pellet[% by mass] Ni Fe₂O₃ SiO₂ CaO Al₂O₃ MgO Others 1.6 53.3 14.0 5.4 3.2 5.7 Binder, carbonaceous reducing agent

Subsequently, the drying-treated pellet was transferred to the preheating chamber 30 provided continuously to the drying chamber 20 and the pellet was subjected to a preheating treatment while the temperature in the preheating chamber 30 was maintained in a range of 700° C. or higher and 1280° C. or lower.

Subsequently, the preheating-treated pellet was transferred to the inside of the rotary hearth furnace 1 and subjected to a reduction treatment and a temperature maintenance treatment. Specifically, as the rotary hearth furnace 1, a region 10 in which the hearth rotationally moves was divided into four to provide four treating chambers, among the four treating chambers, the treating chambers 10 a to 10 c were used as reducing chambers in which the reduction treatment is performed and the treating chamber 10 d was used as a temperature maintaining chamber in which the reduced product is maintained at a high temperature.

In the hearth of the rotary hearth furnace 1, in order to prevent the sample from being difficult to recover when the sample reacts with the hearth and is not peeled off, from the viewpoint of suppressing the reaction between the hearth and the sample as much as possible, the hearth was paved in advance with ash (having SiO₂ as a main component and containing a small amount of an oxide such as Al₂O₃ or MgO as other components).

Incidentally, in Examples in the aspect in which the treatment of maintaining the reduced product at a high temperature is not performed, the internal temperature of the temperature maintaining chamber was set to 0° C. and the mixture passed only through the temperature maintaining chamber. Further, the reduced product obtained through the reduction treatment or the reduction treatment and the temperature maintenance treatment was transferred to the cooling chamber connected to the rotary hearth furnace 1, cooled rapidly to room temperature while allowing nitrogen to flow, and then taken out into the air. Incidentally, the recovery of the reduced product from the rotary hearth furnace was performed in the form of the reduced product being transferred to the cooling chamber 40, and the reduced product was recovered along a guide installed at the cooling chamber 40 by the guide.

Conditions of the reduction treatment and the temperature maintenance treatment in the reduction treatment step are presented in the following Table 4.

Further, the nickel grade of the sample taken was analyzed by an ICP emission spectroscopic analyzer (SHIMAZU S-8100 model) and the metallized rate of nickel and the nickel content rate in the metal were calculated, respectively. Incidentally, the metallized rate of nickel was calculated by the following Equation (i) and the nickel content rate in the metal was calculated by the following Equation (ii).

Metallized rate of nickel=amount of metallized Ni in pellet÷(amount of entire Ni in pellet)×100 (%)   (i)

Nickel content rate in metal=amount of metallized Ni in pellet (total amount of metallized Ni and Fe in pellet)×100 (%)   (ii)

Further, the recovered samples were pulverized by wet treatment and then the metal (ferronickel metal) was recovered by magnetic separation. Then, the recovery rate of Ni metal was calculated from the Ni content rate and the charged amount of the nickel oxide ore charged, and the amount of the recovered Ni. Incidentally, the recovery rate of Ni metal was calculated from the following Equation (iii).

Recovery rate of Ni metal=amount of recovered Ni÷+(amount of ore charged×proportion of Ni contained in ore)×100   Equation (iii)

TABLE 4 High temperature Recovery Reducing Reducing maintaining Metallized Content of rate of temperature time temperature rate of Ni Ni in metal metal Sample (° C.) (Min.) (° C.) (%) (%) (%) 1 1210 55 0 98.0 18.0 90.0 2 1255 45 0 98.2 18.2 90.2 3 1300 35 0 98.8 18.4 90.4 4 1350 18 0 99.3 18.6 90.6 5 1395 14 0 99.7 18.8 90.7 6 1445 8 0 99.7 19.0 90.9 7 1230 50 1310 99.3 18.5 90.6 8 1230 50 1400 99.8 18.9 91.1 9 1230 50 1480 99.9 19.3 92.2

As understood from Table 4, by the mixture containing the source material ore being subjected to the reduction treatment step having at least the drying step, the preheating step, the reduction step in which the reduction is conducted using a rotary hearth furnace, a hearth of which rotates, and the cooling step in which the obtained reduced product is cooled, ferronickel of a high nickel grade could be obtained, and nickel at a high recovery rate of 90% or more as the recovery rate could be recovered.

Further, by performing the reduction treatment or the reduction treatment and the temperature maintenance treatment using the rotary hearth furnace, the internal temperature of the rotary hearth furnace could be maintained to a high temperature, energy required for reheating was suppressed, and an efficient smelting treatment could be performed.

EXPLANATION OF REFERENCE NUMERALS

-   1 ROTARY HEARTH FURNACE -   10 REGION -   10 a, 10 b, 10 c, 10 d TREATING CHAMBER (REDUCING CHAMBER,     TEMPERATURE MAINTAINING CHAMBER) -   20 DRYING CHAMBER -   30 PREHEATING CHAMBER -   40 COOLING CHAMBER 

1. A metal oxide smelting method comprising a reduction treatment step including: a drying step in which a mixture obtained by mixing a metal oxide and a carbonaceous reducing agent is dried; a preheating step in which the dried mixture is preheated; a reduction step in which the preheated mixture is reduced using a rotary hearth furnace, a hearth of which rotates; and a cooling step in which the obtained reduced product is cooled.
 2. The metal oxide smelting method according to claim 1, wherein the reduced product obtained through the reduction step is subjected to a temperature maintenance step in which the reduced product is maintained at a prescribed temperature in the rotary hearth furnace, and after maintained for a prescribed time, the reduced product is supplied to the cooling step.
 3. The metal oxide smelting method according to claim 2, wherein the rotary hearth furnace has a structure capable of partitioning the respective steps, and a treatment in the reduction step and a treatment in the temperature maintenance step are performed using the same rotary hearth furnace.
 4. The metal oxide smelting method according to claims 2, wherein the reduced product is maintained at a temperature of 1300° C. or higher and 1500° C. or lower in the temperature maintenance step.
 5. The metal oxide smelting method according to claim 1, wherein in the reduction step, reduction is performed while a reducing temperature is set to 1200° C. or higher and 1450° C. or lower.
 6. The metal oxide smelting method according to claim 1, wherein the mixture to be dried in the drying step is obtained through a mixing treatment step in which at least a metal oxide and a carbonaceous reducing agent are mixed to obtain a mixture, and a pretreatment step in which a treatment of forming the obtained mixture into a lump product or a treatment of filling the mixture in a prescribed container is performed.
 7. The metal oxide smelting method according to claim 1, further comprising a separating step in which the reduced product cooled in the cooling step in the reduction treatment step is separated into a metal and slag and the metal is recovered.
 8. The metal oxide smelting method according to claim 1, wherein the metal oxide is nickel oxide ore.
 9. The metal oxide smelting method according to claim 1, wherein the reduced product contains ferronickel.
 10. The metal oxide smelting method according to claims 3, wherein the reduced product is maintained at a temperature of 1300° C. or higher and 1500° C. or lower in the temperature maintenance step.
 11. The metal oxide smelting method according to claim 2, wherein in the reduction step, reduction is performed while a reducing temperature is set to 1200° C. or higher and 1450° C. or lower.
 12. The metal oxide smelting method according to claim 3, wherein in the reduction step, reduction is performed while a reducing temperature is set to 1200° C. or higher and 1450° C. or lower.
 13. The metal oxide smelting method according to claim 2, wherein the mixture to be dried in the drying step is obtained through a mixing treatment step in which at least a metal oxide and a carbonaceous reducing agent are mixed to obtain a mixture, and a pretreatment step in which a treatment of forming the obtained mixture into a lump product or a treatment of filling the mixture in a prescribed container is performed.
 14. The metal oxide smelting method according to claim 3, wherein the mixture to be dried in the drying step is obtained through a mixing treatment step in which at least a metal oxide and a carbonaceous reducing agent are mixed to obtain a mixture, and a pretreatment step in which a treatment of forming the obtained mixture into a lump product or a treatment of filling the mixture in a prescribed container is performed.
 15. The metal oxide smelting method according to claim 2, further comprising a separating step in which the reduced product cooled in the cooling step in the reduction treatment step is separated into a metal and slag and the metal is recovered.
 16. The metal oxide smelting method according to claim 3, further comprising a separating step in which the reduced product cooled in the cooling step in the reduction treatment step is separated into a metal and slag and the metal is recovered.
 17. The metal oxide smelting method according to claim 2, wherein the metal oxide is nickel oxide ore.
 18. The metal oxide smelting method according to claim 3, wherein the metal oxide is nickel oxide ore.
 19. The metal oxide smelting method according to claim 2, wherein the reduced product contains ferronickel.
 20. The metal oxide smelting method according to claim 3, wherein the reduced product contains ferronickel. 